Three-dimensional (“3D”) endoscopy can assist with a variety of minimally invasive procedures by providing clinicians with depth information. Achieving depth-resolved imaging having a large, three-dimensional field of view can be difficult when small diameter flexible imaging probes such as, e.g., borescopes, laparoscopes, and endoscopes are utilized. The use of confocal imaging through a fiber-bundle using a high numerical aperture lens may be one technique that can be used to address this problem. Such technique is described in, e.g., Y. S. Sabharwal et al., “Slit-scanning confocal microendoscope for high-resolution in vivo imaging,” Appl. Opt. 38, 7133 (1999). A 3D field of view for such devices, however, may be limited to less than a few millimeters due to a small clear aperture of the objective lens and a low f-number that may be required for high-resolution optical sectioning.
Other techniques such as, for example, stereo imaging and structured illumination have also been proposed for obtaining 3D endoscopic images. Such techniques are described in, e.g., M. Chan et al., “Miniaturized three-dimensional endoscopic imaging system based on active stereovision,” Appl. Opt. 42, 1888 (2003); and D. Karadaglic et al., “Confocal endoscope using structured illumination,” Photonics West 2003, Biomedical Optics, 4964-34, respectively. These techniques may, however, require more components to construct a probe than would be required for confocal imaging that is performed using a fiber bundle. This additional hardware can increase the size, cost, and complexity of such devices.
Spectrally-encoded endoscopy (“SEE”) techniques can utilize a broadband light source and a diffraction grating to spectrally encode reflectance across a transverse line within a sample. For example, a two-dimensional image can be formed by slowly scanning this spectrally-encoded line. This technique can be performed using a single optical fiber, thereby enabling imaging through a flexible probe having a small diameter. In particular, SEE images can have a larger number of resolvable points, and may be relatively free from pixilation artifacts as compared with images obtained using fiber-bundle endoscopes.
When combined with interferometry techniques and systems, SEE can provide three-dimensional images. A depth-resolved imaging can be achieved, e.g., by incorporating a SEE probe into a sample arm of a Michelson interferometer. Using such an arrangement, two-dimensional (“2D”) speckle patterns can be recorded using a charge-coupled device (“CCD”) camera at multiple longitudinal locations of a reference mirror. Subsequently, depth information can be extracted by comparing interference signals obtained at consecutive reference mirror positions. When using this technique, the reference mirror can be held stationary to within an optical wavelength while a single image (or line) is being acquired to avoid the loss of fringe visibility. Scanning a reference mirror that is positioned with such accuracy over multiple discrete depths can be very difficult at the high rates required for real-time volumetric imaging.